Optical probe for wafer testing

ABSTRACT

A first optical probe is used to test a planar lightwave circuit. In one embodiment, a second probe is used in combination with the first probe to test the planar lightwave circuit by sending and receiving a light beam through the planar lightwave circuit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The described invention relates to the field of optical circuits. In particular, the invention relates to an optical probe for testing an optical circuit.

[0003] 2. Description of Related Art

[0004] Optical circuits include, but are not limited to, light sources, detectors and/or waveguides that provide such functions as splitting, coupling, combining, multiplexing, demultiplexing, and switching. Planar lightwave circuits (PLCs) are optical circuits that are manufactured and operate in the plane of a wafer. PLC technology is advantageous because it can be used to form many different types of optical devices, such as array waveguide grating (AWG) filters, optical add/drop (de)multiplexers, optical switches, monolithic, as well as hybrid opto-electronic integrated devices. Such devices formed with optical fibers would typically be much larger or would not be feasible at all. Further, PLC structures may be mass produced on a silicon wafer.

[0005]FIG. 1 is a schematic diagram that shows an example of the current way that planar waveguides 20, 22 are tested. Typically, a PLC wafer is diced and optical fibers, 10, 12 are mounted to the edge of a PLC die. Light is sent into the PLC structure 5 by light source, such as a laser, coupled to a first optical fiber 10, and a photodetector coupled to a second optical fiber 12 detects the power of light transmitted to it. A photodetector coupled to the second optical probe 12 will detect the power of light transmitted to it.

[0006] If the PLC works properly, then optical fibers are permanently attached to the PLC, and the PLC is put into a package. However, if the PLC does not work properly, the unit is discarded, and the time and effort to dice, fiber mount and to comprehensively test the device are wasted. Thus, a method of testing a planar lightwave circuit at the wafer level or before fiber attach is important.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a schematic diagram that shows an example of the current way that planar waveguides are tested.

[0008]FIG. 2 is a cross-sectional schematic diagram of an optical probe used to test a planar lightwave circuit (PLC).

[0009]FIG. 3 is schematic diagram of one embodiment of a prism having a bottom portion that includes a waveguide.

[0010]FIG. 4 is a schematic diagram that shows an example of a PLC die having a first probe region and a second probe region.

[0011]FIG. 5 is a schematic diagram that shows a cross-section view of an optical probe coupled to waveguides taken along their direction of propagation in the PLC.

DETAILED DESCRIPTION

[0012] A method of testing a planar lightwave circuit is achieved by positioning an optical probe in a probe region over a waveguide. In one embodiment, the probe region comprises a waveguide core layer that has either no upper cladding deposited yet, or has a very thin layer of upper cladding deposited. In another embodiment, the probe region has had its upper cladding at least partially removed, e.g., by etching. The remaining upper cladding may be approximately 2 microns or less. In some cases part of the waveguide's core layer may also be removed. A second probe may be used in combination with the first probe to test the planar lightwave circuit by sending and receiving a light beam through the planar lightwave circuit.

[0013]FIG. 2 is a cross-sectional schematic diagram of an optical probe used to test a planar lightwave circuit (PLC) 30. The PLC 30 comprises a waveguide having a core layer 40 and lower cladding 52.

[0014] An optical probe 80 is coupled to the PLC 30 in a probe region 60 having either a thin layer of upper cladding 50 or no upper cladding over the waveguide core. In one embodiment, the probe region 60 may include approximately 1-2 microns of upper cladding 50 over the waveguide core 40. However, if reducing optical loss is important, a thicker upper cladding 50 may be employed.

[0015] In one embodiment, the optical probe 80 is a prism having a rounded top 82 that serves as a lens to direct light incident upon the optical probe's upper surface to be focused toward the bottom portion of the optical probe 80. The probe's upper surface may be either the complete focusing optics or a part of the focusing optics used to couple light between the probe and light source and/or detector. Preferably the optical probe 80 is made of a material harder than that of which it will probe, so that the optical probe will not be scratched during its usage, and can be re-used on other PLCs.

[0016] The optical probe has a slightly higher index of refraction than the waveguide for which it will probe. For example, a high density glass or sapphire may be used to probe a silica waveguide, and lithium niobate (LiNbO₃) or rutile may be suitable for probing a silicon nitride waveguide. The angle of the probe 30 and the probe's index of refraction are selected to match the guided mode of the waveguide of the PLC 30. Different probes may be used for different waveguides.

[0017] In one embodiment, a second optical probe 90 is coupled to a second probe region 92. The second optical probe 90 may be used in combination with the first optical probe 80 to test a waveguide in the PLC 30. In one embodiment, a light source is coupled to the first optical probe 80, and a photodetector is coupled to the second optical probe 90. If the waveguide is working properly the detector will detect light being emitted through the PLC 30. An optical index-matching fluid 70 may optionally be used in the interface between the PLC and the optical probes 80, 90 to improve optical coupling.

[0018]FIG. 3 is schematic diagram of one embodiment of a prism 100 having a bottom portion that includes an integrated waveguide 110. The waveguide 110 may be diffused into the prism from the bottom, created by ion exchange or ion implantation, or may be deposited, e.g., by applying a chemical vapor deposition (CVD) technique. In one embodiment, the integrated waveguide 110 has a slightly higher refractive index than the rest of the probe, and is not uniform over the entire bottom portion of the prism 100. Instead, the waveguide 110 has either an abrupt end, a graded end such as that of a diffused waveguide, or the waveguide 110 may have a grating. The waveguide 110 should have a slightly higher index of refraction than the waveguide for which it will probe, and the waveguide 110 should facilitate a phase velocity that closely matches that of the PLC waveguide. The closeness of the match depends on the remaining thickness of the upper cladding; the smaller the upper cladding thickness, the less accurate a match is needed. The integrated waveguide 110 allows for better coupling of the guided mode of the prism with that of the waveguide of the PLC.

[0019]FIG. 4 is a schematic diagram that shows an example of a PLC die 200 having a first probe region 210 and a second probe region 212. After testing is complete, the optical probes are removed from the PLC die 200. In one embodiment, one or more of the probe regions, such as probe region 210, are removed from the PLC die 200 by slicing the die, e.g., along line 220. In another embodiment, the probe regions are not removed; however, they may be filled with optical index-matching fluid to reduce optical losses in the PLC.

[0020]FIG. 5 is a schematic diagram that shows a cross-section view of an optical probe 300 coupled to waveguides 310, 320 taken along their direction of propagation in the PLC. FIG. 5 illustrates that an optical probe 300 can be coupled to more than one waveguide without moving the optical probe. If a light source is coupled to the optical probe, then the light can be directed to either of the waveguides 310, 320 depending on the angle of incidence of the light into the optical probe. Alternatively, if a photodetector is coupled to the optical probe, then depending on the angle of the photodetector to the optical probe, light can be detected from either of the waveguides 310, 320.

[0021] Multiple waveguides may be integrated into the optical probe similar to the integrated waveguide 110 of FIG. 2 to improve coupling to the waveguides 310, 320.

[0022] A segmented optical probe, i.e., a probe having a top surface with several focuses, may be used. This allows coupling to multiple waveguides at the same time. The optical probe may also comprise a microlens array.

[0023] In addition to the testing methods previously mentioned, this technology can be used for fault isolation or intermediate device debugging capabilities. It can be applied to a whole wafer as well as previously diced and possibly fiber interfaced PLCs if they are found non-optimal in performance. One or more probes with detection and/or transmission capability may be coupled at intermediate positions within the PLC (which would be inaccessible by conventional methods) to measure characteristics of PLC subunits and hence determine the local cause of observed effects for debug, fault isolation, and performance enhancement purposes. In one embodiment, the optical probes may be used with a moderately thick upper cladding. In this case, once the optical probe is removed, the transmission in the PLC is normal, and no loss is due to the temporary placement of the optical probe from testing.

[0024] Thus, a method and apparatus for testing a planar lightwave circuit using an optical probe is disclosed. However, the specific embodiments and methods described herein are merely illustrative. Numerous modifications in form and detail may be made without departing from the scope of the invention as claimed below. The invention is limited only by the scope of the appended claims. 

What is claimed is:
 1. A method of testing a planar lightwave circuit comprising: creating a first probe region of a top surface of the planar lightwave circuit by removing a portion of cladding from the top surface of the planar lightwave circuit; coupling a first optical probe to the first probe region; and testing an optical pathway within the planar lightwave circuit by transmitting or receiving light through the first optical probe.
 2. The method of claim 1 further comprising: creating a second probe region of the top surface of the planar lightwave circuit; coupling a second optical probe to the second probe region; and using the second optical probe in combination with the first optical probe to send and receive a light signal through the planar lightwave circuit.
 3. The method of claim 2 further comprising: changing an input angle of the light signal to test a second optical pathway within the planar lightwave circuit without moving the first optical probe or the second optical probe.
 4. The method of claim 1 further comprising: using an index-matching fluid as an interface between the first optical probe and the first probe region.
 5. The method of claim 1 further comprising: removing the first probe region from the planar lightwave circuit.
 6. The method of claim 1 further comprising: filling in the first probe region from the planar lightwave circuit with index-matching fluid.
 7. A method of testing a planar lightwave circuit having first and second surface regions, the first and second surface regions having an upper cladding thickness of approximately 2 microns or less, the method comprising: coupling a first optical probe to the first surface region; directing light through the first optical probe into the planar lightwave circuit; coupling a second optical probe to the second surface region; and receiving the light through the second optical probe.
 8. The method of claim 7, wherein at least one of the first and second surface regions is near an edge of a planar lightwave circuit die.
 9. The method of claim 7, wherein directing light through the first optical probe further comprises: directing light through a rounded top portion of the first optical probe.
 10. The method of claim 9 further comprising: directing light into a first waveguide in a bottom portion of the first optical probe.
 11. The method of claim 10 further comprising: directing light into a second waveguide in the bottom portion of the first optical probe by changing an input angle of the light.
 12. The method of claim 7 further comprising: using an index-matching fluid as an interface between the first optical probe and the first surface region.
 13. The method of claim 7, wherein testing the planar lightwave circuit is performed on a wafer prior to dicing the wafer.
 14. The method of claim 7, wherein testing the planar lightwave circuit is performed on a die prior to permanently attaching optical fibers to the die.
 15. The method of claim 7, wherein testing the planar lightwave circuit is performed on a die after permanently attaching optical fibers to the die.
 16. An optical probe comprising: a prism having a rounded top; and a first waveguide in a bottom portion of the prism, the rounded top to focus light entering the prism into first waveguide.
 17. The optical probe of claim 16, wherein the prism is at least partially made of sapphire, high density glass, LiNbO₃, or rutile.
 18. The optical probe of claim 16, further comprising: a second waveguide in the bottom portion of the prism, wherein the rounded top constitutes more than one focus to couple light into the first waveguide and the second waveguide.
 19. The optical probe of claim 16, wherein light entering the rounded top is redirected approximately 90 degrees by the prism and the first waveguide.
 20. The optical probe of claim 16, wherein the rounded top comprises a microlens array.
 21. A method of making an optical probe, the method comprising: forming a lens surface on a prism; and forming a waveguide in a bottom portion of the prism.
 22. The method of claim 21, wherein the waveguide is formed by diffusion or ion exchange.
 23. The method of claim 21, wherein the waveguide is formed by ion implantation.
 24. The method of claim 21, wherein the waveguide is formed by deposition.
 25. The method of claim 21 further comprising: forming a second waveguide in the bottom portion of the prism.
 26. The method of claim 21, wherein forming the lens surface on the prism further comprises forming a lens surface having more than one focus.
 27. The method of claim 21, wherein forming the lens surface on the prism further comprises forming a lens surface having a microlens array. 